1. Field of the Invention
The present invention relates to a femoral prosthesis for applying to a human body.
2. Prior Art
Conventionally, artificial hip joints have been composed of a socket inserted into the pelvic bone and a femoral prosthesis which has a stem and a spherical head at the end capable of rotatably engaging the socket.
Generally, the stem, made of a metal such as stainless steel or cobalt-chromium alloy, is inserted into a femoral canal and fixed by using cementing compound between the stem and the canal wall. On the acetabular side, the socket is cemented to a pelvic bone, which receives the spherical head of the stem.
Particularly, the stem is inserted deeply within the elongated canal of the femur, and when the artificial joint is loaded, some difference in Young's modulus between the metal stem and the bone tissue causes different amounts of then deformation, then leading to a sinking of the stem into the bone due to relaxation between the cement compound and the bone or between the metal stem and the bone. This results in losing joint function through looseness between the members or, in the extreme case, separation of the stem form the femoral cavity.
Attempts have been made to prevent the looseness between the cement and bone or the metal stem. For example, the stem material has been made from to titanium or titanium alloy instead of stainless steel or Co--Cr alloy, to approximate the Young's modulus of the stem to that of the bone tissue. This attempt failed to completely resolve the looseness problem.
Another improvement has been attempted, in which the stem is formed such that the outside of the stem approaches to inner dimensions of the, to make the gap between the stem surface and the bone canal as narrow as possible and, then inserting the stem into the bone without use of any cementing compound. It was, however, difficult to configure the stem in a shape to accurately duplicated to the inner canal profile. Practically, sufficient products, except in the case of custom-ordered products, have not been available because of patients having great difference in inner femoral profiles.
Recently, in another attempt, U.S. Pat. No. 4,895,571 describes a interlocking femoral prosthesis utilizing surgical screws which pass through the openings formed through the stem, which fix the stem directly to the femoral cavity without use of the cement material.
The screw comprises a straight portion and a wider threaded portion with a greater length than the straight portion, and, on the other hand, the opening formed within the prosthesis stem is set to have a inner diameter wide enough to pass the threaded portion of the screw therethrough, resulting in loose gaps between the screw and the inner wall of the opening. This method can cause severe mechanical instability problems of relative movement, such as micro-movement, of the stem with regard to the bone.